1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device that is adaptable for use in large, high picture quality displays. The present invention also is directed to a method of fabricating such a liquid crystal display device.
2. Description of the Related Art
Generally, an active matrix liquid crystal display (LCD) uses thin film transistors (TFT's) as switching devices to produce a natural-looking moving picture. Since such LCDs can be made smaller than existing Cathode Ray Tube (CRT) displays, they have become widely used as monitors for devices such as personal computers, notebook computers, office equipment, cellular phones, and pagers.
Referring now to FIG. 1, which is a plan view showing a partial structure of a conventional liquid crystal display device, that LCD includes a TFT that is arranged extended from the intersection of a data line 4 and a gate line 2 to a portion of the data line 4. A pixel electrode 22 is connected to the drain electrode 10 of the TFT.
The TFT includes a gate electrode 6 that is connected to the gate line 2, a source electrode 8 that is connected to the data line 4, and the drain electrode 10. The source electrode 8 projects over the gate line 2 and is a part of the data line 4. The data line 4 is applied with a gap to the semiconductor layer 14 and 16. The drain electrode 10 and the pixel electrode 22 are connected together via a contact hole 20. The TFT also includes semiconductor layers 14 and 16 that define a channel 24 between the source electrode 8 and the drain electrode 10 when a gate voltage is applied to the gate electrode 6. Referring now to FIG. 2, the channel 24 typically has a length L of about 5 to 6 μm and a width W of about 25 μm. Referring now back to FIG. 1, the channel is arranged along the data line 4 so as to increase the ON current. In operation, a TFT responds to a gate signal on the gate line 2 by selectively applying a data signal on the data line 4 to the pixel electrode 22.
In practice, an LCD includes a plurality of TFTs and pixel electrodes 22. Each pixel electrode 22 is positioned in a cell area defined between data lines 4 and gate lines 2. The pixel electrodes are comprised of a transparent conductive material. Each pixel electrode 22 can produce a potential difference with a common transparent electrode (not shown) provided on an upper substrate (not shown). By way of a potential difference, a liquid crystal positioned between the lower substrate 1 (not shown) and the upper substrate (not shown) is rotated due to dielectric anisotropy in the liquid crystal. Thus, a molecular arrangement of the liquid crystal varies according to the voltage that is applied to each pixel electrode. As a result, the light transmission through the liquid crystal display varies pixel by pixel such that video information is displayed.
FIG. 3A to FIG. 3E are plan views and sectional views, taken along line a–a′ of FIG. 1, which are useful for explaining a method of fabricating an LCD device as shown in FIG. 1. Referring first to FIG. 3A, the gate line 2 and the gate electrode 6 are provided on a substrate 1. The gate electrode 6 is formed by depositing a metal, such as aluminum (Al) or copper (Cu), such as by sputtering, and then etching to form the gate line and gate electrode.
Referring now to FIG. 3B, an active layer 14 and an ohmic contact layer 16 are then fabricated on a gate insulating film 12 formed on the gate line 2 and the gate electrode 6. The gate insulating film 12 is typically formed by depositing an insulating material, such as silicon nitride (SiNx) or silicon oxide (SiOx), by plasma enhanced chemical vapor deposition (PECVD). The active layer 14 is then fabricated from undoped amorphous silicon, while the ohmic contact layer 16 is formed from amorphous silicon that is heavily doped with an n-type or p-type impurity. The active layer 14 and the ohmic contact layer 16 are fabricated by deposition and subsequent patterning using chemical etching.
Referring now to FIG. 3C, the data line 4, the source electrode 8 and the drain electrode 10 are then provided on the gate insulating film 12. The data line 4 and the source and drain electrodes 8 and 10 are typically made from chrome (Cr) or molybdenum (Mo). The source electrode 8 projects over the gate line 2 and is a part of the data line 4. The data line 4 and the source and drain electrodes 8 and 10 are formed by first depositing a metal layer (i.e., Cr or Mo) using chemical vapor deposition or sputtering and then patterning that metal layer. After the data line 4, source electrode 8, and drain electrode 10 are formed, the ohmic contact layer 16 over the gate electrode 6 is patterned to expose the active layer 14. The area over the gate electrode 6 between the source and drain electrodes 8 and 10 in the active layer forms a channel 24.
Referring to FIG. 3D, a protective layer 18 and a contact hole 20 are then added to the structure. The protective layer 18 and the contact hole 20 are formed by depositing an insulating material over the exposed surface. The protective layer is then patterned to form the contact hole 20 that exposes the drain electrode. The protective layer 18 is beneficially made from an inorganic insulating material such as silicon nitride (SiNx) or silicon oxide (SiOx), an acrylic organic compound, or an organic insulating material having a small dielectric constant such as Teflon, BCB (benzocyclobutene), Cytop or PFCB (perfluorocyclobutane).
Referring now to FIG. 3E, a pixel electrode 22 is then formed on the protective layer 18. The pixel electrode 22 is formed by depositing a transparent conductive material, such as indium-tin-oxide (ITO), indium-zinc-oxide (IZO) or indium-tin-zinc-oxide (ITZO), on the protective layer 18 and into the contact hole 20. The transparent conductive layer is then patterned. The pixel electrode 22 is electrically connected, via the contact hole 20, to the drain electrode 10.
In such LCD devices, the gate electrode 6 extends along the data line 4, as shown in FIG. 1, so as to enlarge the channel 24. When a typical photo resist pattern is used to fabricate an extended gate electrode 6, the uniformity of the resulting gate electrode is not particularly good as the photo resist deteriorates during fabrication. The result can be a defective gate electrode and consequent reduced manufacturing throughput. Additionally, an LCD device having an extended gate electrode has a problem in that, since the TFT area extends along the data line to correspond to the lengthy gate electrode, the display area has a reduced aperture ratio.
In addition, as the overlapping area A of the gate electrode 6 and the data line 4 increases (see FIG. 1), the parasitic capacitance Cgs, which is proportional to the overlapping area A, increases. A large parasitic capacitance Cgs can cause flicker and a residual image, both of which reduce picture quality. Also, a large parasitic capacitance reduces the speed of response such that acceptable large-dimension LCDs are not possible.